Dielectric heating apparatus



Filed April 12, 1968 March 24,1970 c. E. M.TIBBS 3,502,839

DIELECTRIC HEATING APPARATUS 4 Sheets-Sheet 1 Inventor W57)? 2% %YW March 24, 1970 c. E. M. TIBBS DIELECTRIC HEATING APPARATUS 4 Sheets-Sheet 2 Filed April 12, 1968 II I. II

MK 2 m A I. v mp w yw Wm 6 March 24, 1970 c. E. M. TlBBS DIELECTRIC HEATING APPARATUS Filed Ap ril 12, 1968 4 Sheets-Sheet '5 Inventor 7M g mp a i /W24 Attorney 2 March 24, 1970 c. E, M. T1885 3,502,839

DIELECTRIC HEATING APPARATUS Filed April 12, 1968 4 Sheets-Sheet 4 72 74 N Kfi ,78 40 w fa'i Inventor Attorneys United States Patent 3,502,839 DIELECTRIC HEATING APPARATUS Christopher Evan Mundell Tibbs, Wokingham, England, assiguor to Dysona Industries Limited, Wokingham, Berkshire, England, a British company Filed Apr. 12, 1968, Ser. No. 720,961 Claims priority, application Great Britain, Apr. 18, 1967, 17,817/67, 17,818/67; Apr. 24, 1967, 18,798/67; May 18, 1967, 23,090/67; July 24, 1967, 33,878/67, 33,880/ 67; Aug. 1, 1967, 35,245/ 67, 35,246/ 67; Oct. 11, 1967, 46,436/67 Int. Cl. H05b 9/06 US. Cl. 219-1055 14 Claims ABSTRACT OF THE DISCLOSURE A microwave oven has a vertically sliding door and has door guides formed to wedge the door towards the oven face in the final portion of its closing movement to reduce the escape of RP. energy. At the end of a heating cycle, a solenoid is energised to overcome the friction due to the wedging and any other forces holding the door closed but the solenoid lifts the door through only a small part of its upward travel after which a counterbalancing spring completes the door opening movement. The door has some freedom of movement in relation to the counterbalancing spring to ensure that the termination of its upward movement is not abrupt.

In microwave ovens for heating food, it is vconvenient to have a door to provide access to the oven from its front rather than a lid at its top. For reasons of space, a sliding door is preferable, particularly if it has an up-and-down direction of sliding.

An important requirement in dielectric heating apparatus is that the escape of radio-frequency energy shall be as small as possible. To ensure that the door, when closed, makes close contact with the front wall of the oven, it is customary for a sliding door to have a wedging action such that the door is pushed into close contact with the front face of the oven in the final portion of the door closing motion. However, although this helps to reduce the escape of electrical interference, the opening of the door requires a considerable manual effort, especially if a sealing gasket of conductive rubber has been incorporated between the door and the oven face for the further reduction of the escaping electrical interference. The manually exerted force cannot be stopped at the instant that the wedging action releases and consequently the door is frequently pulled very rapidly to its upper limit of movement and suffers considerable mechanical shock. This mechanical shock tends to cause misalignment of the door, resulting in escape of electrical interference and in further difficulties in releasing the door from its wedging position.

According to the present invention, in an oven in which the door slides in guides in a vertical direction between its open and closed positions and in which the guides are formed to wedge the door towards the oven face in the final portion of its closing movement, there are additionally counterbalancing means for assisting in opening the door, means for holding the door in its closed position when the oven is in operation, means for automatically terminating the application of microwave energy to the oven at the end of the heating cycle, and a solenoid for energisation at the end of the heating cycle, the solenoid being arranged to exert on the door a force sufiicient to overcome initial resistance to motion but to move the door upwards for less than the distance required for complete opening. On energisation of the solenoid, the door is forced upwards out of its wedged position by the soleice noid, which is thus responsible for the initial part of its opening travel, and thereafter the counterbalancing means permit the movement to continue until the door reaches its fully open position.

In the preferred arrangement of our apparatus, the solenoid is arranged to pull upwards a plate which lifts the door away from its fully closed position and the plate is spring-loaded in an upward direction so that it maintains contact between the plate and the door as soon as the door is raised by the solenoid. We have found that this reduces chatter between the plate and the door and imparts a smoother movement to the door. A further feature of our preferred arrangement is an electric switch which is operated as a consequence of the movement of the door through a predetermined distance and which then opens the energising circuit of the solenoid. One practical advantage of this is that the force exerted by the solenoid need be enough only to initiate the door movement and that the rating of the solenoid need be only for intermittent operation of very short duration. Consequently, the solenoid can be much smaller than would be required if it were responsible for the whole of the motion of the door between its closed and open positions. A further advantage is that the counterbalance need only be enough to ensure the continuance, after the de-energisation of the solenoid, of the upward movement of the door. In an oven incorporating a wedging action in the final portion of the door closing motion, the door guides may be designed to allow the door to travel very freely for the remainder of its sliding movement. Thus, the counterbalance has little resistance to motion to overcome, apart from the weight of the door.

As a consequence of these features, the door does not reach its open position with excessive momentum. As a further measure to avoid mechanical shocks to the door, we support the door at each side on blocks attached to the counterbalance lifting tapes so that the doors have some freedom of movement in the direction of travel..The blocks are of small weight compared with that of the door and the stops corresponding to the open position of the door are arranged in the paths of the blocks. Consequently, when the door is opened the blocks are brought to a halt by contact with these stops but the door is not obstructed and continues to move upwards for a short distance before falling back on to the blocks at the end of the counterweight tapes. There is thus no significant impact to cause misalignment of the door at the end of the opening position.

In the preferred arrangement, the guides for the door are so formed that the door is freely removable in the upward direction. This is of considerable practical importance because the door includes a perforated inspection panel (the perforations being of such a size that they do not normally permit the escape of a substantial amount of radiation) and frequent cleaning of this panel is desirable. During the operation of the microwave oven, fats will be splashed on to the perforations and will alter the characteristics of the perforated panel. This may cause microwave energy to be radiated, to the danger of the operator. To facilitate the removal and easy re-location of the door, it is provided on each side with vertical tongues which pass through holes in the stops attached to the counterbalances.

According to a further subsidiary feature, we arrange that the control for switching on the microwace energy is mounted behind the path of the door as it moves to its open position. Thus, once the door begins to move at the end of the heating cycle, when the solenoid is energised, there is no access to the control for switching on the microwave energy. In ovens having, for example, hinged doors, it is convenient to close the doors when not in use and it is then easy to press the heating control unintentionally and thus to initiate a heating period with the oven empty. This may result in damage to the food shelf and to the door seal. With an oven having an automatic ally opening upwardly sliding door, the door when open does not obstruct working space in the region around the oven and consequently it is natural to leave it pen from the end of one heating operation until the oven has been re-loaded for the next heating operation. By placing the controls in such a position that access to them is prevented once the door begins to open, we make it necessary to perform two successive operations independently (the closure of the doors and the operation of the control made accessible by the door closure) before a heating cycle is initiated. It is most unlikely that both operations will be performed with the oven empty.

As has been stated above, the tight closure of the door ensured by the wedging action assists in preventing excessive escape of stray radio-frequency radiation. In many microwave ovens, when the door is closed a thin sheet of metal (for example of aluminum or stainless steel) is affixed to the marginal edge of the inner surface of the door and presses against the metallic face plate of the oven so as to make good metallic contact all round the door. Usually, rubber is fixed behind the metal sheet to provide a resilient contact. However, this arrangement has the disadvantage that if a particle of food is trapped between the contacting metal surfaces, sparking will occur. Sparking may also occur if the face plate is dented or damaged so that a small air gap is produced between the metal surfaces. Another proposal is to place a gasket of resistive rubber around the periphery of the door to absorb radio-frequency radiation and to provide the conductive contact between the door and the oven. This also suffers from the disadvantage of sparking if particles are trapped between the contacting surfaces and moreover results in a high coefficient of friction between the door and the oven face plate. We have discovered that a layer of low friction plastics material on the surface of the resistive rubber reduces the friction between the tWo closure surfaces without significantly increasing the microwave interference leakage. This plastics material may be pianted on the surface of the resistive rubber to form a coating and may then be cured with the coating. It may, for example, consist of polytetrafluoroethylene with a resin binder. Alternatively, a low friction nylon extrusion may be glued to the surface of the resistive rubber. Molybdenum disulphide is preferably incorporated in the nylon extrusion in order to provide it with an extremely hard non-shaving surface and a very low frictional coefficient.

It is in some cases possible to dispense entirely with the resistive rubber, a layer of low-friction plastics material having a very low electrical loss factor being formed directly on the door or on the oven frame around the door opening so that it lies between the oven frame and the door when the door is in its closed position. The door should include on its. inner surface, inwards of the layer of plastic material, a groove forming a microwave choke section. The capacitance resulting from the layer of plastics material between the oven face and the rim around the door, alone or with the microwave choke section, presents to the microwave current a very low impedance. This plastic layer may advantageously be applied to the door by dipping the door frame, formed with the choke section, into the plastics material which is to form the layer and by heating the door to the temperature required to cause the plastics material to adhere to the door. The door may for example be heated to a high temperature and then dipped into a fluidised bath of the plastics powder.

In order that the invention may be better understood, one example will now be described with reference to the accompanying drawings, in which:

FIGURE 1 is a perspective view of the oven with the door in its closed position and FIGURE 1A is a similar view of one corner of the oven front face with the door slightly raised;

FIGURE 2 is a side view of a part of the oven with the side wall partly broken away and with the door n its open position;

FIGURE 3 is a plan view of the door guide on one side of the oven;

FIGURE 4 is a rear elevation of the oven door, removed from the oven;

FIGURE 5 illustrates the operation of the counterbalancing springs;

FIGURE 6 is a section through a marginal portion of the door;

FIGURE 7 shows the solenoid and contacts on the other side of the oven adjacent the door guide; and

FIGURE 8 is a diagram of a simple door control circuit.

The general appearance of the oven is shown in FIG- URE 1. A door 10 closes an opening in the front of the oven through which food to be heated is inserted into the oven. The door includes a perforated panel 12 permitting inspection of the contents, the perforations being of such a size that they do not permit the escape of radio-frequency energy. The door is arranged for vertical sliding motion between its open and closed positions.

Above the door, controls 14 and indicator lamp 15 are recessed into the front face 16. The controls 14 permit the selection of the time for which the food will be heated and the initiation of the heating cycle once the door has been pressed down to its closed position. The lamp 15 when illuminated indicates that a heating cycle is in progress. The heating cycle is automatically terminated at the end of the period set by the buttons 14 and, as will be described later, the door is then automatically opened. It will be clear that, in opening, the door slides over the controls and thereby prevents the initiation of a further heating cycle until the door is again closed.

FIGURE 1A illustrates the position of a permanent magnet 17 at the left-hand bottom corner of the oven. Theer is a second permanent magnet at the right-hand bottom corner. These magnets act on the mild steel door frame through non-magnetic stainless steel lip (which constitutes a bottom stop for the door) and thus hold the door in its closed position.

FIGURES 2 and 3 show in side elevation and plan view respectively the guide members 18 and 20 which define the vertical movement of the door. As shown in FIG- URE 3, the ends of these guide members are covered with extruded sections 22 and 24 of low friction plastics material. The guide members exist on both sides of the oven and the door has on each side a member which slides between the extrusions 22 and 24.

As shown in FIGURES 2, 4 and 5, short side walls 26 extend perpendicularly from the front wall of the door 10 and strips 28, parallel to the front wall, extend from the rear edges of the side walls 26. These strips 28 are the guided members which enter between the extruded sections 22 and 24 of the guide members on each side of the oven body.

It will be seen from FIGURES 4 and 5 that each strip 28 is formed with a tongue 30 at the inner portion of its lower edge. This tongue passes through a slot 32 (FIGURE 3) in a block 34 to which is attached the end of a steel tape 36 emerging from a counterbalance box 38. The box 38 houses a coiled counterbalancing spring, the inner end of which is fixed and the outer end of which is connected to the tape 36. The spring will exert a pull on the tape 36 and therefore on the block 34, and the block 34, in moving upwards, will carry with it the sliding door. There is a counterbalancing spring and block 34 on each side of the oven.

It will be seen from FIGURE 2 that the front face 16 of the oven is not parallel with the guiding slot between the guide members 18 and 20. The front wall of the door is arranged at asimilar angle with respect to the guide strips 28 at the rear of the door, the spacing between the front wall and the strips 28 being such that the rear face of the front wall of the door will be a very close fit against the front face 16 of the oven body when the door is in its lowermost position. Because of the angled faces, as the door moves downwards (with the members 28 in th guide slots), the front wall of the door will be pulled against the front face of the oven and the portion of the side wall of the oven between its front face 16 and the guide slot will act as a wedge between the rear face of the front wall of the door and its guided member 28.

As has been explained above, the wedging action is very beneficial from the point of view of ensuring a close fit and reducing the escape of electrical interference, but it has the disadvantage that the door is difficult to move from its closed position and a manually exerted force to release the door may result in imparting excessive velocity to the door throughout its travel. To overcome this, we arranged that when the predetermined heating period has reached its end and the microwave energy is cut off, a solenoid 40 (FIGURE 2) acts to release the door from its wedged position but is incapable of carrying the door through more than a short portion of its total travel between the closed and open positions. The solenoid 40 has a plunger 42 at the end of which is a plate 44. In FIGURE 4, the plate 44 is shown in full lines in the position which it assumes when the door is open and in dotted lines in its position when the door is closed. A spring 46 nor mally urges the plate 44 to the upper position, but this spring exerts much too small a force to be able to open the door without energisation of the solenoid. The purpose of the spring 46 is simply to maintain contact between the plate 44 and the base of the block 34 as the door moves upwards following energisation of the solenoid. Without this spring there is intermittent contact between the plate and the block with any energising supply except fully smoothed DC. This intermittent contact is undesirable firstly because the door opening movement is less smooth and secondly because it is very noisy.

Thus, when the door is moved from its open to its closed position by manual effort it carries with it the block 34 and the block 34 depresses the plate 44 to the position shown in dotted lines in FIGURE 2. In the closed position, the door is wedged against the front face of the oven and is additionally held by a permanent magnet. At the end of the heating cycle, the solenoid 40 is energised and the plate 44 imparts to the base of the block 34 an upward force sufficient to release the door. The plate 44 carries the door through the first half-inch or inch of its travel, but stops at the position shown in full lines in FIGURE 2. The effect of the permanent magnets on the door, once the door has been lifted by the solenoids from its closed position, is negligible. The counterbalance spring then ensures that the upward motion of the door is continued.

We have already referred to the use of conductive rubber strip around the inner face of the front wall of the door. When the door is in its closed position this rubber strip is squeezed against the front face of the oven. Consequently, in ovens employing such a strip and also employing the wedging feature described above, the movement of the door out of its closed posit-ion becomes even more diflicult. However, we have discovered that the friction between the door and oven face can be reducedwithout substantially increasing the microwave leakage. To achieve this, we place a coating 48 of plastics material on the rubber extrusion 50 (FIGURES 4 and 6). This rubber extrusion is attached to an aluminium flange 52 around the margin of the inner face of the door.

The flange 52 forms part of an aluminium section which incorporates the microwave choke 54. This is a recess having an opening on to the inner face of the door and of such dimensions that it tends to absorb microwave energy at the frequency for which the oven is designed to operate. The choke recess may be filled with, for example, alumina cement or foamed polyurethane. A low-friction skin of polytetrafiuoroethylene is then deposited and cured over the whole of the inner surface of the door, including the conducting rubber surface. This gives a smooth door surface which is easy to wipe clean.

Because the high friction between the door and oven face has been reduced by the plastics layer and because the solenoid need only accelerate the door through the first portion of its upward travel and is energised only for a brief interval, the dimensions of the two solenoids can be quite small and they can be comfortably housed behind the side walls in the oven shown in the drawings. A sole noid powerful enough to throw the door from its closed to its open position would be impracticable for an oven of this kind.

In the arrangement shown in the drawings, the plate 44 at the side of the oven shown in FIGURE 2 is also used to control the position of a pivoted bar 60. The purpose of the bar 60 is to provide an additional safeguard to ensure that the oscillator is not still working with the door open. The automatic timer is responsible for the opening of the door and for the operation of a first switch to break the circuit to the H.T. contactor, which cuts off the supply to the oscillator and therefore cuts off the high frequency energy in the oven. Although such devices fail very rarely, failures have been known, for example due to the welding of the H.T. contactor terminals. In FIG- URE 2, the post 64 is connected to the H.T. circuit and the contact 66 is earthed through a resistor (not shown) within the bar 60 and through the spring 62. Thus, when, with the door in its closed position, an energising pulse is applied to the solenoid, the plate 44 forces the door out of its closed position and, 'as it rises, permits the spring 62 to bring the earthed contact 66 into electrical connection with the terminal 64. Thus, if the H.T. contactor has not removed H.T. from the circuit, the H.T. circuit will be earthed through the contact 66. In the arrangement shown, the bar 60 carries a second contact 68 which, when the bar is in the position shown in chain-dotted lines, is resiliently forced away from the contact 66. When the door opens, the contact 68 makes connection with the contact 66 shortly after the latter contact has made connection with the H.T. terminal 64. The contact 68 is connected to earth through a trip relay which is energised by the voltage across the resistor incorporated in the bar 60. The trip relay opens a contact in the circuit of the H.T. transformer primary and this contact then latches open so that it does not close when the trip relay is deenergised. The incorporation of this trip relay in the circuit prevents repeated short-circuiting of the H.T. supply to earth by the repeated opening and closing of the door with the equipment in a faulty condition. When the door is in its open position, the spring 46 holds the plate 44 in its upper position and thus the bar 60 remains in the position shown in full lines until the door is again closed.

On the other side of the door, a similar solenoid plate 44 (FIGURE 7) controls two microswitches 70 and 72. The function of these switches will be apparent from FIGURE 8. The normally open microswitch 72 is closed by the closure of the door. The microswitch 70 is normally closed. The switch 74 in the timer 7-6 is closed when one of the timing buttons 14 is pressed. Each of the buttons 14 defines a different period. While switches 72 and 74 are closed, the coil 78 for the main H.T. contactor is energised. After a period defined by the selected button 14, the switch 74 opens automatically, thereby de-energising the coil 78, and a normally open switch 80 closes to complete the circuit for the solenoid 40, the switch 80 being then held closed by a latching circuit. The solenoid lifts the door and as soon as the plate 44 has reached the position shown in FIGURE 7 it opens the microswitch 70, thereby breaking its own energising circuit. This also results in the de-energisation of the above-mentioned latching circuit and the release of contact 70.

I claim:

1. A microwave oven comprising:

a door which, in its closed position, covers the front face of the oven;

door guides coacting with said door and permitting sliding motion of said door in a vertical direction between its upper open and lower closed positions, said door guides being formed to wedge the door towards the oven face in the final portion of its closing movement;

counterbalancing means for assisting in opening said door; means for holding said door in its closed position; means for automatically terminating the application of microwave energy at the end of the heating cycle;

and a solenoid device for energisation at the end of the heating cycle, arranged to move the door upwards for less than the distance required for complete opening with a force sufficient to overcome initial resistance to motion;

whereby on energisation of the solenoid device the door is forced upwards by the solenoid device for the initial part of its opening travel and thereafter the counterbalancing means permit the movement to continue until the door reaches its fully open position.

2. A microwave oven in accordance with claim 1, in which the solenoid device includes a solenoid and a plate, which, when the solenoid is energised, lifts the door away from its fully closed position, the plate being spring-loaded in an upward direction to maintain contact between said plate and said door as the door is raised by the solenoid.

3. A microwave oven in accordance with claim 1, including an electric switch operated as a consequence of the movement of said door through a predetermined distance to open the energising circuit of said solenoid dev1ce.

4. A microwave oven in accordance with claim 1, in which the means responsive to the end of the heating cycle to cut off the microwave energy also energises said solenoid device to open said door.

5. A microwave oven in accordance with claim 4, comprising oven controls so arranged above the door that they are covered by the door in its fully open position.

6. A microwave oven in accordance with claim 1, in which said counterbalancing means acts on the door through a terminal device which is of small weight compared with that of the door, the door having some freedom of movement in its direction. of travel with respect to the said terminal device, the oven including a stop so arranged in the ath of the terminal device t a it obstructs the terminal device, but does not prevent continued upward movement of the door.

7. A microwave oven in accordance with claim 6, including on each side of the door a counterbalancing spring to the end of which is attached an apertured block constituting said terminal device, the door having on each side a prong which passes through the aperture in the corresponding block.

8. A microwave oven in accordance with claim 6, in which the door rests on said terminal device so as to be freely movable in an upward direction and the door guides are open at their upper ends to permit removal of the door for cleaning.

9. A microwave oven in accordance with claim 1, in which the door has a low friction surface layer consisting at least partly of plastics material, where it meets the front face of the oven in its closed position.

10. A microwave oven in accordance with claim 9, in which the door has a gasket of resistive rubber around its periphery, where it meets the oven face, to absorb interference radiation, and in'which a layer of low friction plastics material is formed on the surface of the resistive rubber.

11. A microwave oven in accordance with claim 9, including a layer of low friction plastics material applied directly to the metal surface of the door where the door meets the front face of the oven when in its closed position.

12. A micromave oven in accordance with claim 11, in which the plastics material is applied by dipping the door into a bath of the plastics material and heating the door to the temperature required to cause the plastics material to adhere to the door.

13. A microwave oven in accordance with claim 12, in which the layer of plastics material is formed on the door by dipping the latter into a fluidised bath of the plastics powder.

14. A microwave oven in accordance with claim 9, including a channel formed in the inner surface of the door, extending all round the edge of the inner panel and inwards of the line at which the door meets the oven face,

. said channel being filled with an insulating material and forming a microwave choke section, and in which the inner surface of the door, including the choke-filling material, is covered with a coating of plastics material.

References Cited UNITED STATES PATENTS 2,632,090 3/1953 Revercomb et al. 219--10.55 2,827,537 3/1958 Haagensen 2l9-10.55 3,249,731 5/1966 Johnson 219-1055 3,335,656 8/1967 Smith 21910.55 X

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner 

